Pd-Modified ZnO-Au Enabling Alkoxy Intermediates Formation and Dehydrogenation for Photocatalytic Conversion of Methane to Ethylene.

ABSTRACT

Photocatalysis provides an intriguing approach for the conversion of methane to multicarbon (C2+) compounds under mild conditions; however, with methyl radicals as the sole reaction intermediate, the current C2+ products are dominated by ethane, with a negligible selectivity toward ethylene, which, as a key chemical feedstock, possesses higher added value than ethane. Herein, we report a direct photocatalytic methane-to-ethylene conversion pathway involving the formation and dehydrogenation of alkoxy (i.e., methoxy and ethoxy) intermediates over a Pd-modified ZnO-Au hybrid catalyst. On the basis of various in situ characterizations, it is revealed that the Pd-induced dehydrogenation capability of the catalyst holds the key to turning on the pathway. During the reaction, methane molecules are first dissociated into methoxy on the surface of ZnO under the assistance of Pd. Then these methoxy intermediates are further dehydrogenated and coupled with methyl radical into ethoxy, which can be subsequently converted into ethylene through dehydrogenation. As a result, the optimized ZnO-AuPd hybrid with atomically dispersed Pd sites in the Au lattice achieves a methane conversion of 536.0 µmol g-1 with a C2+ compound selectivity of 96.0% (39.7% C2H4 and 54.9% C2H6 in total produced C2+ compounds) after 8 h of light irradiation. This work provides fresh insight into the methane conversion pathway under mild conditions and highlights the significance of dehydrogenation for enhanced photocatalytic activity and unsaturated hydrocarbon product selectivity.